The present invention generally relates to apparatus and methods for drying samples under conditions of elevated temperature and reduced pressure. More particularly, it relates to a new and improved chemical vapor trap for use in a vacuum drying system which is effective to selectively remove water, acetic acid and other materials known to be harmful to vacuum pump operation and service life by isolating these materials from the drying vapor phase and blanketing them under a chemical blanket to prevent their undesired revaporization during the remainder of the sample drying operation.
Vacuum drying instruments are presently known and used for various scientific purposes. Illustrative examples include gel dryers, specimen dryers, rotary evaporators, vacuum centrifuges and filtration manifolds. The sample may be a biological specimen or fluid or it may be a treated substrate material to be used in subsequent analytical determinations. The samples generally contain a solvent portion or fraction to be removed which typically includes water, acids, organic liquids and other solvents. The solvents are removed in these devices by applying heat and vacuum to a drying chamber containing the samples. The liquid and vapor forms of most of these solvents are known to be deleterious to the vacuum pumps used for creating a partial vacuum in the drying chambers.
More particularly, oil sealed rotary vane vacuum pumps are used to create a vacuum in a sample chamber. Oil sealed rotary vane vacuum pumps are preferred because of the good pumping speeds and rapid and efficient ability to dry a sample. A major drawback previously associated with rotary vane pumps is that heated water vapor and acetic acid condense in the pump in the oil case resulting in an acetic acid, water and oil emulsion. In rotary pumps wherein the lubricating oil is fed from the oil case, the water settles at the bottom of the pump case. At start up, instead of receiving oil lubricant, the pump sees acidified water which is non-lubricating when the condensed solution reaches the oil feed hole. Often what happens is that after start up, the pumps actually freeze up from lack of lubricant. Efforts to prevent entry of heated liquids into the pump or condensation of heated acidified water in the pump have included placing a liquid trap in line before the vacuum pump to trap and remove liquid phase materials evolving from the heated sample. The liquid traps are generally effective to prevent large amounts of liquids from traveling directly from the sample to the pump, however, a liquid trap cannot prevent vapor phase contaminants such as acetic acid and water vapor from entering the pump. Moreover, at lower pressures the trapped liquids may volatilize and add to the corrosive vapor load feeding to the pump which is undesirable.
Prior attempts to reduce or eliminate vapor phase corrosive contaminants have included placing a cold trap between the liquid trap and the vacuum pump to cause water and acetic acid to condense out of the vapor phase feed to the vacuum pump. The vapor phase load at the beginning of a drying cycle is often too large for the condenser capacity so that contaminated vapors still reach the pump in an undesirably high concentration. Moreover, solvent flashing and fly-by can occur in the cold trap permitting hot corrosive vapors also to exit the cold trap in a high concentration. Furthermore, cold traps are maintenance intensive requiring setup for each drying session and the cost of refrigerants is an undesirable extra expense.
In view of the problems associated with premature failures and problematic maintenance of oil sealed rotary vane pumps, a compromise has been made by employing lower speed, less efficient diaphragm vacuum pumps, because they are generally more resistant to vapor phase composition. Liquid phase contaminants carried over into a diaphragm vacuum pump or condensed from vapor phase materials in the pump itself cause damage to the diaphragms and premature failure of the pumps. In the past, diaphragm pumps were believed to provide an agreeable alternative to rotary vane vacuum pumps because the need for a cold trap could be dispensed with. A liquid trap is generally used to collect liquid phase contaminants. At lower system pressures achieved by vacuum pump evacuation, the liquid in the trap revaporizes into the vapor phase which may condense in the pump and cause premature diaphragm failure. Often people have suggested the use of a cold trap in a diaphragm pump system which represents an unacceptable compromise because the maintenance issues concerning the cold trap are not avoided and the pump has a slower pumping rate leading to slow, i.e. unacceptably long, drying times.
Other efforts to alleviate or avoid solvent contamination of the vacuum pumping devices in drying systems, have included the suggestions made in U.S. Pat. No. 5,137,604 and U.S. Pat. No. 5,025,571. In the first of these patents, a cold trap is employed in connection with an elaborate switching device for opening the cold trap and sample chamber to vacuum in discrete episodes. This proposed improvement incrementally passes evolving vapor phase materials through the cold trap to increase the residence time of the vapor phase in the condenser to more effectively remove the corrosive contaminants from the vapor phase prior to entry into the pump. The vapor phase and liquid phase spill over from most samples is so high at the beginning stages of the drying cycle that incomplete condensation is the norm, so that contaminants still reach the pump and can condense therein and cause damage. More importantly, the drying times of the samples are undesirably long and cold trap maintenance is not avoided.
In U.S. Pat. No. 5,025,571, another method of preventing liquid egress comprises collecting liquids and condensed vapors in a heated liquid trap which effectively revaporizes all evolving solvent to heated vapor phase form before entering the diaphragm pump. Pre-heating still does not and cannot prevent the undesirable condensation of harmful corrosive liquids within the pump itself which is a disadvantage, particularly in view of the compromise already made in terms of pumping speed and drying times for these diaphragm pump systems.
Accordingly, to overcome the disadvantages of prior art vacuum drying systems, it is an object of the present invention to provide a new and improved chemical vapor trap which is generally effective to selectively substantially remove water and/or acetic acid contaminants from a vapor phase in a vacuum drying system, regardless of vacuum source, to prevent these corrosive materials from entering the vacuum pump.
It is another object of the present invention to provide a new and improved chemical vapor trap which is effective to selectively remove water and acetic acid from an evolving vapor phase in a vacuum drying system by condensing these materials into a liquid phase condensate and isolating the condensate from the vacuum system in a manner which prevents re-vaporization and reintroduction of the corrosive acid and water materials into the vapor phase being fed to the vacuum pump.
It is another object of the present invention to provide a new and improved vacuum drying system for drying samples which is faster, and more efficient, at drying sample materials in a relatively maintenance-free manner as compared to prior art vacuum drying systems.